The present invention relates to a WDM (Wavelength Division Multiplexing) transmission system which transmits a plurality of signal lights having wavelengths different from each other, a WDM transmission repeater provided in an optical transmission line in the WDM transmission system, and a WDM transmission method in such a WDM transmission system.
By transmitting a WDM signal group including a plurality of signal lights with wavelengths different from each other through an optical fiber line as a transmission line, a WDM transmission system enables high-speed, large-capacity optical communications. It enables large-capacity optical communications, e.g., 32 channels (utilizing 32 signal lights) at 2.5 Gb/s, even in the case where an existing single-mode optical fiber line network is employed. As a consequence, the introduction of WDM transmission system has recently been under way in order to respond to rapid increases in communication demands such as those in Internet or the like.
In such a WDM transmission system, there are cases where a repeater base station having a WDM transmission repeater is disposed somewhere in the transmission line from a transmitter within a transmitter base station to a receiver within a receiver base station. The WDM transmission repeater is equipped with an optical amplifier, an optical ADM (Add-Drop Multiplexer), and the like. The optical amplifier functions to collectively optically amplify the WDM signal group propagating through the transmission line. The optical ADM functions to demultiplex the WDM signal group into a first group of signal lights and a second group of signal lights different from those of the first group, receive the second group of signal lights, multiplex a separate third group of signal lights with the first group of signal lights, and send the resulting new WDM signal group into the transmission line again.
As a result of studies of the conventional WDM transmission systems thus configured, the inventors have found the problems as follows.
Namely, the conventional WDM transmission systems and WDM transmission repeaters are designed so as to be applicable to long-haul main lines, so that the distance between adjacent repeaters is long, e.g., on the order of 60 km to 80 km, in such a WDM transmission system. As a consequence, the optical amplifier within each WDM transmission repeater is required to have such properties as a wide band and high gain, a minimum gain deviation (fluctuation in gain between individual signal lights having their respective wavelengths), and the like in order to collectively compensate for the transmission loss of the whole signal lights propagating between the repeaters having such a long distance therebetween. However, not only such a high-performance optical repeater is expensive, but also its reliability is hard to secure.
In the local inter-station transmission, by contrast, the distance between the adjacent repeaters is relatively short, i.e., a few km to ten plus several km, and the received signal light level at each repeater is high, whereby it is often unnecessary to utilize optical amplifiers having a high gain or any optical amplifiers at all. Also, in general, since signal lights are received and transmitted by each repeater, it is often unnecessary for all the signal lights having wavelengths different from each other to be optically amplified with a uniform gain. Consequently, the conventional WDM transmission repeaters for long-haul main lines, equipped with high-performance expensive optical repeaters, are not suitably applicable to local interstation WDM transmission systems in terms of cost and equipment. It is not so economical to apply such conventional WDM transmission repeaters to the local inter-station WDM transmission system as in the case of applying the WDM transmission system with a long-haul main line.
In order to overcome the problems such as those mentioned above, it is an object of the present invention to provide a WDM transmission system suitable as optical communication means between local stations, a WDM transmission repeater suitable for the WDM transmission system, and a WDM transmission method in such a WDM transmission system.
The WDM transmission system according to the present invention comprises a plurality of WDM transmission repeaters, whereas the interval between these WDM transmission repeaters is not greater than 20 km, thereby enabling optical communications suitable for relatively short inter-station transmission.
In such a WDM transmission system, it is unnecessary to use a wide-band optical amplifier which can amplify all of a plurality of signal lights, and a narrow-band amplifier which can optically amplify a small number of demultiplexed signal lights is sufficiently applicable thereto. Here, the distance between the respective center wavelengths of the individual signal lights is not greater than a few nm. Specifically, there are transmission modes of 1.6 nm, 0.8 nm, 0.6 nm, and the like. As a consequence, though the number of employed WDM transmission repeaters, i.e., the number of optical amplifiers, increases, these optical amplifiers can utilize inexpensive products, and the optical amplifiers become unnecessary in some instances, whereby the system as a whole can be realized at a low cost. Also, since the distance between the repeaters is relatively short, i.e., 20 km or less, no high output is required for the optical amplifiers employed, whereby a sufficient degree of reliability is secured. In addition, since only a small number of signal lights in the WDM signal group propagating through the transmission line are optically amplified by each WDM transmission repeater, the risk of all the channels (all the signal lights) failing to establish communications is low in the event of failure of an optical amplifier in any of the WDM transmission repeaters.
The WDM transmission repeater according to the present invention has a configuration suitable for the above-mentioned WDM transmission system and is installed in a transmission line through which a WDM signal group including a plurality of signal lights having wavelengths different from each other within a usable wavelength band propagates. The WDM transmission repeater comprises an optical ADM having an entrance end for receiving the WDM signal group from the transmission line; a first port for taking out, from first and second groups each composed of one or more signal lights and separated from the WDM signal group taken in by way of the entrance end, the second group of signal lights; a second port for receiving a third group of signal lights composed of one or more signal lights within the usable wavelength band, each having a wavelength different from those of the first group; and an output end for sending to the transmission line a new WDM signal group including the first group of signal lights and the third group of signal lights taken in by way of the second port.
Also, the WDM transmission repeater according to the present invention may be configured so as to comprise, in addition to the optical ADM, at least one of a first optical amplifier for amplifying the second group of signal lights taken out from the first port of the optical ADM and a second optical amplifier for amplifying the third group of signal lights to be received from the second port of the optical ADM.
In a configuration such as that mentioned above, the taken-out second group of signal lights is amplified by the first optical amplifier, whereby a sufficient receiving sensitivity is assured in the WDM transmission repeater at the next stage. On the other hand, as the newly introduced third group of signal lights is amplified by the second optical amplifier, the light intensity of the crosstalk component in each of the signal lights in the not-demultiplexed second group can be lowered relative to the light intensity of the WDM signal group (mainly including the first and third groups of signal lights). Namely, it becomes more likely to determine that signal lights in the second group are signal lights each having a level not higher than a permissible crosstalk level at which the WDM transmission repeater at the next stage does not receive the second group of signal lights as a significant optical signal.
Further, the WDM transmission repeater according to the present invention may be configured so as to comprise a demultiplexer (included in a wavelength separating device) having an input port, optically connected to the first port of the optical ADM, for receiving the second group of signal lights from the first port, and at least one output port, each prepared for evry signal light in the second group taken in by way of the input port, for taking out a signal light associated therewith; and a multiplexer (included in a wavelength multiplexing device) having at least one input port, each prepared for every signal light in the third group to be newly introduced to the transmission line, for receiving a signal light associatedd therewith, and an output port, optically connected to the second port of the optical ADM, for sending the third group of signal lights to the second port. In such a configuration, the first and second ports of the optical ADM may be connected to the input port of the demultiplexer and the output port of the multiplexer, respectively, either directly or by way of the first and second optical amplifiers, respectively.
In a configuration such as that mentioned above, each of the demultiplexer and multiplexer is preferably provided with a structure for enabling attachment and detachment of a respective end portion of a branch line for optically connecting a selected output port in the output ports of the demultiplexer to a selected input port in the input ports of the multiplexer, in order to effectively respond to an increase or decrease in the number of subscribers to be connected. In this case, a selected output port in the output ports of the demultiplexer has a first joint structure for enabling attachment and detachment of a first end of the branch line for optically connecting the selected output port of the demultiplexer to a selected input port in the input ports of the multiplexer. Also, a selected input port of the multiplexer has a second joint structure for enabling attachment and detachment of a second end of the branch line opposed to the first end thereof.
In this configuration, when one selected output port of the demultiplexer and one input port of the multiplexer are optically connected to each other by way of a bypass line, part of the signal lights (included in the second group) taken out from the output port of the demultiplexer is taken into the multiplexer from the input port thereof by way of the bypass line, so as to constitute part of the third group of signal lights. When this bypass line is removed from between the demultiplexer and the multiplexer, each of the wave number of signal light receivable by the WDM transmission repeater and the signal lights transmittable thereby would increase. Consequently, wave number control can easily be carried out in response to an increase or decrease in subscribers for each repeater base station, as the bypass line is attached thereto and detached therefrom.
Further, the WDM transmission repeater according to the present invention may have a configuration which can tolerate a certain degree of propagation of a crosstalk component in the signal lights (in second group) to be taken out (a relatively inexpensive optical ADM having a low demultiplexing performance can be used). Namely, this WDM transmission repeater has a structure in which at least two optical ADMs having the same function are optically connected to each other by a bridge line. In this configuration, the optical ADM at the first stage for receiving the WDM signal group from the transmission line does not use the port for inputting the third group of signal lights. Also, in the optical ADM at the second or later stage for sending a new WDM signal group into the transmission line, the port for taking out the second group of signal lights from the received WDM signal group is not used (the optical ADM at the second or later stage functioning as an optical filter for blocking the second group of signal lights in this case). As a plurality of optical ADMs optically connected to each other by way of the bridge line are utilized to realize the original function of the optical ADM, the crosstalk component of the second group of signal lights to be taken out can be reduced to a receiving sensitivity level in the WDM transmission repeater at the next stage or lower (i.e., permissible crosstalk level or lower).
Here, the WDM transmission repeater equipped with a plurality of optical ADMs as mentioned above may also comprise the above-mentioned demultiplexer and multiplexer, and the demultiplexer and multiplexer can be provided with a structure for enabling attachment and detachment of a bypass line for optically connecting one selected output port in the demultiplexer and a selected input port in the multiplexer to each other. Also, the WDM transmission repeater may be configured such that a first optical amplifier is installed between the demultiplexer and the port for taking out the second group of signal lights for amplifying the second group thus taken-out of signal lights, whereas a second optical amplifier is installed between the multiplexer and the port for receiving the third group of signal lights for amplifying the third group of signal lights.
In a WDM transmission repeater having a structure suitable for a WDM transmission system which can be realized by various kinds of structures mentioned above, the WDM transmission repeater being selected from a plurality of WDM transmission repeaters in the WDM transmission system, the WDM transmission method according to the present invention is characterized in that the signal level of each signal light in the second group is individually adjusted so as to satisfy both of a first condition that it exceeds a receiving sensitivity level of the WDM transmission repeater at an entrance end thereof and a second condition that it is lower than a receiving sensitivity level of the adjacently downstream WDM transmission repeater at the next stage at an entrance end thereof.
Preferably, in the configuration such as that mentioned above, signal level adjustment for the second group of signal lights is carried out in the WDM transmission repeater (including the transmitting station), positioned upstream, for amplifying the second group of signal lights. Namely, the WDM transmission repeater positioned upstream separately amplifies the individual signal lights in the second group so that their respective signal levels satisfy both the above-mentioned first and second conditions.
In this WDM transmission method, the respective signal levels of the individual lights in the second group are appropriately adjusted in the WDM transmission repeater (any of the transmitting station and the WDM transmission repeater positioned upstream) sending out the second group. Consequently, of the propagating WDM signal group, each of the individual signal lights in the second group to be taken out by the WDM transmission repeater has a signal level not lower than the receiving sensitivity level of the WDM transmission repeater, so as to be reliably received. Also, of the second group of signal lights to be taken out, the crosstalk component directed to the WDM transmission repeater (any of the receiving station and the WDM transmission repeater positioned downstream) at the next stage is not higher than a permissible crosstalk level, whereby the signal lights to be taken out from the signal lights in the first and third groups is reliably received by the WDM transmission repeater at this next stage.
Further, the WDM transmission method according to the present invention is characterized in that, in a WDM transmission repeater selected from a plurality of WDM transmission repeaters in the above-mentioned WDM transmission system, the center wavelength of each signal light in the introduced third group exists between the center wavelengths of two signal lights adjacent to each other in a plurality of signal lights included in a WDM signal group, and is set so as to not to be lower than a predetermined crosstalk suppressing ratio with respect to each of the two wavelengths.
Specifically, in the WDM transmission system according to the present invention, it is preferred that the respective intensities of signal lights in the first group at the respective center wavelengths of signal lights in the third group be set lower than the respective peak intensities of the signal lights in the first group by 25 dB or more. Also, the respective center wavelengths of signal lights in the third group are preferably set so as to exist between the respective center wavelengths of two signal lights adjacent to each other in the second group of signal lights, whereas the respective intensities of the signal lights in the second group at the respective center wavelengths of signal lights in the third group are set so as to be lower than the respective peak intensities of signal lights in the second group by 10 dB or more. In any of these cases, the crosstalk component of each signal light in the second group is kept from affecting the system.
Here, the respective center wavelengths of signal lights in the third group may be identical to the respective center wavelengths of signal lights in the second group, totally different from the respective center wavelengths of signal lights in the second group, or exist between the respective center wavelengths of received signal lights (included in the WDM signal group that has propagated through the transmission line). Also, each of the first, second, and third groups includes one or more signal lights.